Damage to the brain, usually defined by dead brain cells which would otherwise survive, is known to be caused by several things:
-fetal alcohol syndrome-being constantly on speed for about 3 days
-large quantities of Ecstasy (a massive overdose)
-incompetently synthesized synthetic drugs
-being constantly drunk for about 3 days
-being hit on the head reeeal hard (see child abuse)
-being raised by fundies
Many people will claim that *any* mind-altering substance causes brain damage, but since this has never been scientifically demonstrated, these people are generally ignorant. See Church of Voodoo Pharmacology (assuming the editors have accepted it).

The general result is retardation, and it can cause the victim to join the fundies or other cults. It can also cause epilepsy, which can increase the ease of acceptance in a fundiecult because many of them view seizures as divine possession.